So what might this dark matter stuff be?

A new analysis of experimental results hints that dark matter may be a …

Amongst astronomers and cosmologists there is no longer much in the way of debate over the existence of dark matter. Micro-lensing observations have turned up results that are nigh on impossible to explain using modified theories of gravity, which themselves need dark matter to explain the cosmic microwave background radiation spectrum. At present there are no viable alternatives, leaving physicists to ponder what dark matter might actually consist of. We know a lot about what properties dark matter particles must have: very weak interaction with electromagnetism, stability, and a rather high formation probability at the time of the big-bang.

Most researchers have pinned their colors to a class of particles called weakly interacting massive particles (WIMPs). These particles are predicted by extensions to the standard model of physics and are expected to be detected by the Large Hadron Collider.

An alternative is mirror matter. The idea of mirror matter comes from symmetry considerations. Basically, if something remains unchanged after a rotation, it has rotational symmetry. Likewise, if something remains unchanged after reflection, it has mirror symmetry. With the exception of the weak force, everything we observe has rotational, translational, and mirror symmetry. For the type of physicist who like beauty (and we all like beauty), this is a rather jarring finding. On the other hand, if every particle has its mirror component, then mirror symmetry is restored and everything looks beautiful once again, providing a strong aesthetic motivation for pursuing this line of thought.

It turns out that mirror particles probably cannot interact directly with their normal counterparts, meaning that they behave exactly like dark matter—in fact, mirror matter interacts more weakly than a WIMP would. Along with the aesthetics of the theory, this provides a relatively parsimonious explanation for dark matter, because we know that it must be stable and we know the masses. On the other hand, it is rather difficult to explain why there is such a huge imbalance between the amount of mirror matter and normal matter.

So which is it, and how could we tell? A small but rather significant set of experimental results may be the first step towards discriminating between the two choices.

In Italy, a group of physicists have been looking for dark matter by capturing the precious few occasions when it does interact non-gravitationally with normal matter in an experiment called DAMA/LIBRA. Essentially, when a WIMP collides directly with the nucleus of an atom, the whole atom will recoil and, in the process, emit a photon. The photon is then detected. However, there are lots of different particles that can set off this reaction, and none of the known particles are dark matter. To distinguish between them, the researchers used the movement of the Earth in relation to then known distribution and motion of dark matter in the galaxy—something that is obtained from observing the motion of normal matter. As the Earth orbits the sun, it will oscillate between moving with and against the movement of the galaxy. When the Earth is moving upstream of the galaxy's motion, more events are expected than when moving with. Earlier this year, the DAMA/LIBRA experimental results were reported and the annual modulation was clearly seen. The signal had exactly the right properties expected for a signal that came about due to an interaction with the Earth and the galaxy as a whole—so even if this isn't dark matter it is something new and exciting.

Unfortunately, the champagne must remain corked for the moment because a number of other experiments, using different detection methodologies, have returned null results. Normally, in such a case, we would simply go for the most sensitive experiment and accept that its results were most likely to correct. However, the different methodologies, different observation times, and different sensitivities to different interaction energies makes it impossible to choose one result over the other.

In this particular case, it could well be that all the experiments are accurate. A researcher in Australia (yes, we can hold that against him) has looked at the data and realized that all the results might be explained by mirror matter. He discovered that only the DAMA/LIBRA experiment had sufficient sensitivity in the right energy regime to detect mirror matter. Furthermore, the observed energy spectrum fits nicely with that predicted by a dark matter halo consisting of mirror hydrogen or helium, which are expected to be the major components of mirror matter.

So is it mirror matter or WIMPS? Both are the result of extensions to the standard model, however, these extensions don't say anything about the stability of WIMPS, while it does tell us that mirror matter is stable. However, for the big bang to look like it did, mirror matter has to have started with an initial temperature much lower than normal matter. Interestingly, that can only be accommodated by certain types of inflation. As a result, some substantial insight into the nature of both dark matter puzzle inflation could be on the horizon with experiments to detect mirror matter set to begin, the large hadron collider providing us with flocks of WIMPs to analyze, and sensitivity improvements in the direct dark matter detectors coming on-line soon.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.